1. Field of the Invention
The invention relates to a pneumatically actuated and hydraulically controlled apparatus for moving a loaded actuator piston at a constant speed, regardless of the magnitude and direction of the actuator piston loading. This synergistic result is achieved by employing a particular form of by-pass valve which automatically ensures a constant volumetric flow rate regardless of the pressure of the by-passing hydraulic fluid or pneumatic pressure variations.
2. Description of the Prior Art
Various and sundry prior art actuators have employed fixed or adjustable valvings for regulating the pressure drop in a hydraulic by-pass circuit. While a constant pressure drop valve will ensure a given volumetric flow rate for any fixed inlet pressure, variations in line pressures result in varying volumetric flow rates.
Exemplary of prior art actuators are the United States patents, as follows:
______________________________________ KONDO 3,929,057 SCHIMMEYER 3,871,527 ROSAEN 3,858,485 POLIZZI 3,850,078 McLELLAND 3,824,900 ALEXANDER 3,807,284 HALLER 3,687,013 HUTTER 3,302,533 SCHOLIN 3,190,077 ERICSON 2,587,449 ______________________________________
The patent to Kondo illustrates a hydraulic brake mechanism which includes a regulating valve to control the flow of hydraulic fluids, for the disclosed purpose of adjusting the speed of the air cylinder piston. Unlike the present disclosure, Kondo employs a needle valve 41 to adjust the rate of actuation speed, and is not directed to a constant actuation speed, regardless of loading.
Schimmeyer employs a regulating valve 75, for allowing an hydraulic override through port 51. As such, Schimmeyer is concerned with maintaining a constant pressure loading and not with a constant speed of actuation for widely varying loads.
The patents to Rosaen and Polizzi both represent additional variable speed hydraulic actuators, and not a combination which purposes are to ensure a constant speed, in response to widely varying loadings. Rosaen employs flow valves 164 and 182 for manually controlling a two-way actuation, at variable speeds. Polizzi employs a hydraulic timer which is controlled by a needle valve 35, for an adjustment of speed response to varying loads.
The patents to McLelland and Alexander both further illustrate variable speed actuators, where the acceleration of the actuator is varied. For this purpose, McLelland supplies air to piston 11 with oil flow controlled by variable bypass valve 64. Again, there is no contemplation of employing a constant volume flow rate to maintain a constant speed. Similarly, Alexander teaches a hydraulic cylinder 24, with hydraulic flow being controlled through passage 106, by needle valve 126. Alexander further employs a stop collar 140 to change the speed of actuation at varying stroke positions, in complete distinction to the purposes of the present invention.
The patent to Haller illustrates a pneumatic-hydraulic actuator employing valve structure formed around a common shaft. Haller, significantly, includes an interchangeable contour part, 40, to control oil flow through orifice 52. Again, Haller is concerned with a variable speed actuator, in complete distinction to the present disclosure.
The patent to Hutter includes a control cartridge 23, for the purpose of metering oil from chamber 50 into reservoir 49, while the present disclosure allows for constant speed over widely varying loads. The patent to Hutter particularly teaches changing the control cartridges 23 when different design loads are to be encountered.
The actuator taught in the patent to Scholin employs an adjustable needle valve 59 for controlling fluid flow between spaces 36b and 37b. The adjustable needle valve 59 of Scholin is particularly taught for changing the pressure of hydraulic fluid within passage 56, and is without any disclosed purpose or ability to maintain a constant hydraulic piston actuation speed in response to widely varying loadings.
Finally, Ericson teaches a structurally unrelated form of hydraulic actuator, wherein regulating valves 110 and 115 are used to control overpressures during a feeding operation. Ericson's device senses the pressure on a milling cutter, and consequently change the speed of work feed. Again, there is no purpose in Peterson for a constant volumetric flow rate valve, to control the by-passing of hydraulic fluid on either side of a hydraulic load piston, regardless of the variation of piston loading.
In summary, there has not been found any teaching in the prior art which recognizes the employment of a constant flow control valve in combination with a hydraulic load piston, as specifically taught in the combination presented herein. Significantly, the present combination includes a pneumatic actuation chamber, for applying actuation pressure against one volume of hydraulic fluid through a free piston, to allow for constant actuation of the hydraulic load piston whenever the pneumatic pressure exceeds a minimum threshold value.